Current-limiting fuses with increased interrupting capacity



F. J. KOZACKA 2,866,038 CURRENT-LIMITING FUSES WITH INCREASED INTERRUPTING CAPACITY Dec. 23, 1958 3 Sheets-Sheet 1 Filed May 9, 1957 1 Emzemwd(r152 znws m, 3w WM W Dec. 23, 1958 F. J. KOZACKA 2,866,038

. CURRENT-LIMITING FUSES WITH INCREASED INTERRUPTING CAPACITY Filed May 9, 1957 3 Sheets-Sheet 2 I?" 6 I g9 ily.

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Dec. 23, 1958 F. J. KOZACKA ,0

CURRENT-LIMITING FUSES WITH INCREASED INTERRUPTING CAPACITY Filed May 9, 1957 3 Sheets-Sheet 5 1:221. T CURRENT I CURRENT Frederick J. Kozacka, South :Hamptn,.N. H., assignor to The Chase-*Shawmut Company, Newburyport, Mass.

Application'May 9;, 1957, Serial No.'658;162

Clain1s. or. 200-120 This invention refers to current-limiting fuses, i. e. to fuses which operate so rapidly that they are capable of interrupting an electric circuit before a fault current of predetermined magnitude can reach its potential peak value, generally referred to as the available short-circuit current of the particular electric circuit.

From a more limited aspect this invention has reference to clearing fuses having a small current-limiting ratio. The term clearing ratio r refers to the ratio of the max. fi -dz from the time of fault inception to the time of current zero resulting from operation of the fuse to 10 times the rated current z}. of the fuse. This ratio may be expressed as follows:

In "order to maintain the clearing ratio as small as possible fault inception must be followed as soon as possible by are inceptiomand arc inception must be followed as soon as possible by a rapid current decay to zero.

Another object of .the invention is to provide current- United States Patent 0 limiting fuses whose clearing ratio is sufiicientlylow to i be applicable for the protection of the most critical static rectifiers, which are static rectifiers provided with germanium diodes, or silicon diodes.

It is another object of the invention to provide current-limitingfuses having a ribbon fuse link for increased current carrying capacity which fuses comprise arc quenching means precluding formation of fulgurites immediately adjacent the point of arc inception, but permitting formation of fulgurites at points remote from the point of arc inception, these fulgurites being spaced by a gaseous gap resulting from burning back of the ribbon link.

Another object of the invention is to provide currentlimiting fuseshaving insulating means interposed between the fuse link and the pulverulent arc-quenching filler by which the fuse link is surrounded wherein said insulating means are adapted to operate as self-adjusting arc chutes 'formingventing passages whose cross-sectional area increases as the pressure generated at the arcing zone increases.

Another object of the invention is to provide currentlimiting fuses which comprise an arc-quenching structure interposed between the arcing zone and a pulverulent arc-quenching filler by which the arcing zone is surrounded wherein the cross-sectional area of said arcquenching structure enabling the escape of products of arcing from the arcing zone to the pulverulent arcquenching filler is relatively large and wherein the access of the pulverulent filler to the arcing zone is impeded.

Still another object of the invention is to provide current-limitingfuses comprising silicious arc-quenching materials arranged immediately adjacent to the are path abstracting large amounts of heat from arcs formed in the fusesby virtue of the high latentheat of fusion which is characteristic of such arc-quenching materials, and in which current-limiting fuses silicious arc-quenching materials are being applied jointlywith organic insulating materials in a form tending to inhibit formation of fulgurites at the. point of arc inception.

Further objects and advantages of the invention will become more apparent as the following descriptionproceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to, and forming part of, this specification.

For a better understanding of the inventionreference may be had to the accompanying drawings wherein Fig. 1 is a longitudinal section of a 'currentdimitin'g fuse embodyingtheinvention;

Figs. 2a and 2b are 'a side'elevation and a front elevation of the fuse link and associated parts of the structure of Fig. 1 shown on a larger scale;

Figs. 3a and 3b are a side elevation and a front e'levation of the structureof Figs. 2a and 2b shortly after the time of arc initiation;

Figs. 4a and 4b are a side elevation'and a front elevation of the structure of Figs. 2a and 2b shortly before the time of arc extinction; I

Fig. 5 refers to another fuse embodying this invention and is a section along 5-5 of Fig. 6;

Fig. 6 refers to the same structure as Fig. '5 and is a section along 6.*6 of Fig. 5;

Fig. 7 is a top plan view showing on a larger scale the residue of the fuse link of the structure of Figs. 5 and 6 and the fulgurite resulting from fusion of quartz sand;

Fig. 8 is a section along 88 of Fig. 7,; I

Fig. 9 is a top plan view of parts of another fuse -,embodying thisinvention;

Fig. 1-0 is a section-of the structure shown in Fig. 9 upon blowing thereof taken along 10-10 of Fig. 1.1;

Fig. 11 is a section taken along 11-41 of Fig. 9; and

Figs. 12:: and "12b are oscillograms showing the operation of current-limiting fuses embodying this invention.

Referring now to the drawings, and more particularly Figs 1 to 4b thereof, numeral 1 has been applied to indicate a tubular casing of insulating material closed on both ends by conductive terminal elements 2 in the form of plugs. Each plug 2 is provided with a blade contact 3 for inserting the fuse into a fuse holder (not shown). Transverse steel pins 4provide a firm mechanical tie between casing 1 and plugs 2'. Each plug 2 is provided with three grooves 20. The radially outer grooves 2a receive insulating plates 5 for spacing plugs 2, whereas the radially inner grooves 2a receive the ends of a ruse link 6. Fuse link 6 is of the ribbon type, as can best be seen from Figs. 2b4b. It'consists of a high conductivity, low fusing energy material, preferably silver; yet copper may be used, if desired. Fuse link 6 is provided with at least one point of reduced cross-sectional area 7 situated between the ends thereof. 'Link 6 is sandwiched between a pair of plates '8. Plates S'a're adapted to abstract large amounts of heat from the are formed at the point of reduced cross-sectional areaj7. To this end plates 8 comprise a silicious material .in highly disperse form and an organic binder for said material. The silicious material may be granular, but is preferably in the form of fibers. It may either be quartz? sand, or fiber glass, embedded in a suitable insulating material. A silicious pulverulent tiller 1!), e. g. quartz sand, is arranged inside of easing 1 and substantially separated from the point of reduced cross-sectional area 7 by the plates 8. means 9 projecting transversely across plates 8. Fastening means 9 may be either formed by rivets, or by eyelets, and the mass thereof ought to be as small as pos Plates 8 are held together by fastening when link 6 is carrying current.

ifulgurite can form in the aforementioned gaps.

amountof heat is abstracted from the arclet 12 which point of reduced cross-sectional area 7 of link 6.

7 Figs. 2a and 2b show the parts 6 and 8 in the normal condition thereof, i. c. When link 6 is carrying current of'a predetermined permissible magnitude. Point 7 of reduced cross-sectional area is sufiiciently short in length formed upon blowing of the fuse in the region of the and small in width to virtually form a point heat source 7 On occurrence of an excess current initial fusion of link 6 and are initiation both take place at the point of reduced cross-sectional area'7. This point 7 is preferably formed by two lateral substantially V-shaped incisions 6 in link 6. When an electric arc is kindled at point 7 some of the products of arc ng are substantially free to escape through V-shaped incisions 6 and the coextensive gaps formed between plates 8. The blasts thus produced tend to remove the few particles of quartz' sand which may have penetrated into the gaps formed between plates 8. As a result, no A large takes theplace of the point of reduced cross-sectional area 7 of link 6. This is due to the fact that plates 8 comprise silicious material in highly disperse form with its attendent high latent heat of fusion. In spite of the fact that such material is present immediately adjacent the arc path the material has little or no tendency to be converted into a conductive fulgurite since the silicious constituents of the material are embedded in a suitable synthetic resin. The embedding resin may be a melamine resin or a SlllCOl'16 resin. The latter family of resins is preferable if the fuse is supposed to operate at relatively high temperatures. In order to absorb large amounts of heat during the arcing time the plates 8 must be in immediate physical engagement, i. e. close heat exchanging relation, with the arc path and the fuse link 8. Eveii silicone resins cannot, without suffering damage, be directly exposed to heat relatively close to the fusing point of silver or copper. It is, therefore, necessary to take proper precautions to preclude the point of reduced crosssectional area or neck 7 from ever reaching the damage temperature of the. synthetic resin binder forming part of plates 8, unless the fuse actually blows, in which case it does no matter much whether or not plates 8 sufier permanent damage. To preclude permanent damage to plates 8 while the fuse is carrying current below the current required for causing blowing of the fuse, i. e. while the fuse is carrying its rated current or but a relatively small harmless overload, the temperature of neck 7 is kept below the danger temperature of the synthetic resin binder of plates 8 by provision of an overlay on neck 7 of a link-destroying low fusing point metal as, for instance, 111 or indium. Such an overlay reacts at its fusing point metallurgically with the base metal of which the fuse link 8 is made, resulting in formation of alloys of the two metals having a high resistivity, and formation of these alloys causes rapid destruction or disintegration of fuse link 8. This metallurgical reaction is well known in the fuse art, and therefore does not need to be de scribed in detail. Suffice it to state that the object of the link-destroying overlay is not to provide a time lag in the operation of the fuse but to protect the plates 8 from temperatures exceeding the danger temperature of the material of which they are made by causing destruction of the link at temperatures below the damage point of plates 8. In order to preclude the overlay on neck '7 from delaying the operation of the fuse, i. e. to increase its pre-arcing times, the thickness of the overlay must be very small, e. g. less than A of an inch.. Such overlays can best be established by electroplating. A very thin overlay of tin on a link of copper or silver may even increase the speed of operation of the fuse, i. e. decrease the pre-arcing times thereof.

After fusion of neck 7 an arc is initiated at the point where neck 7 is situated. The are burns back at a rapid rate, resulting in the build-up of a pressure zone adjacent the arc. The very narrow gap between plates 8 is not sufficient to vent the pressure build-up between plates 8. Since the plates 8 are relatively resilient their axially outer edges 8' are being bent outwardly in a direction transverse to the longitudinal axis of link 6 (see Figs. 3a and 4a). Such outward bending provides some pressure relief, and the gases escaping in a direction longitudinally of link 6 beyond edge 8 are being rapidly cooled down by the pulverulent silico-us or is present adjacent the edges 8'.

Figs. 3a and 3b illustrate this phase of the interrupting process. These figures indicate also the formation of two fulgurites F each adjacent to one of the edges 8'. These fulgurites F are formed remote from the arcing zone, and therefore cannot form a conductive bridge between the juxtaposed back-burning ends of fuse link 6. This eliminates or reduces the danger of restrikes after current zero, a danger which is often present in quartz sand filled current-limiting fuses. I

It is of considerable importance that the are products escaping from the arcing zone are caused to flow preponderantly along link 6 in a direction longitudinally thereof. By virtue of this fact the arc products are cooled by the link and the latter is heated by the arc products. Cooling of the are products by link 6 limits the amount of cooling action required adjacent edge 8' where the arc products are released into the quartz filler It), and thus minimizes the size of fulgurites F. Heating of link 6 by the products of arcing sweeping along link 6 brings the fuse link nearer to its fusing and vaporization temperature, and therefore less are energy is required to produce a predetermined burn-back of fuse link 6. in other words, burn-back and are extinction occur at a more rapid rate.

Figs. 3a and 3b show a relatively short arc between the separated burning back portions of fuse link 8.

Figs. 4a and 4b show the maximum length of are 12 immediately preceding the'extinction thereof. The are has been shown as burning back slightly beyond the axially outer edges 8' of plates 8. This occurs only at extremely severe interruptions of the circuit for whose protection the current-limiting fuse is intended. Normally the arc gap is confined to the space bounded by the edges 8' of plates 8. The escape of are products along the straight line defined by the two rivets 9 is minimized due to the presence of the two rivets 9. This is of considera' ble importance because a substantial escape of arc products along this line tends to cause damage to the casing 1 upon blowing of the fuse due to exposure to excessive heat of the arc-near internal surfaces of the casing 1. This can be effectively avoided by causing the blast escaping from the arcing zone to flow preponderantly in a direction longitudinally of link 6.

The structure shown in Figs. 5 to 8 is in part the same as that illustrated in Figs. 1 to 4b and described in co-nnection with these figures. It is, therefore, sufficient to describe only those details of the structure of Figs. 5 to 8 which differ from the structure of the foregoing figures.

As shown in Figs. 5 to 8 fuse link 6 has a neck 7' or portion of greatly reduced cross-section and supports two circular discs 18' of a fiber-glass-synthetic-resin-laminate which are rivetted together at 9'. The center of discs 18' is arranged in registry with neck 7'. Rivets 9 may consist of a synthetic resin to minimize the amount of metal vapor formed at the arc gap upon blowing of the fuse. Fuse link 6 is surrounded. by a pulverulent arcquenching filler 10', preferably chemically pure quartz quartz sand filler 10 which sand. Quartz sand is substantially kept away from the space enclosed between plates 18' but for a relatively few grains which may enter the interplate space. Upon fusion of neck 7' and are initiation some of the products of arcing are free to blow into the pulverulent fuse filler 10' through the gap formed between plates 18'. That gap is, however, much too small to provide relief for the rapid build-up of pressure at the arcing zone. Nor can the edges of plates 18' situated immediately adjacent to rivets 9 be bent up by the pressure generated between plates 18'. The points to be bent upwardly under the action of internal gas pressure are those situated at the intersections of the circumferences of discs 18' with the longitudinal axis of link 6'. Consequently the amount of are products vented is largest in the direction longitudinally of link 6' and smallest in transverse direction, and decreases progressively from the former direction to the latter. The geometrical configuration of the fulgurites shown in Fig. 7 is clear evidence of the distribution of the quantity of products of arcin'g escaping at each point of the periphery of discs 18.

Referring now to Figs. 9 to 11, inclusive, ribbon fuse link 6' is threaded through a sleeve 12" made of fiber glass and a synthetic resin which separates the individual strands of glass fibers and thus tends to prevent converion of the sleeve 12 under the heat of the are into a more or less homogenous fulgurite which is highly conductive as long as incandescent. Sleeve 12" keeps the main body 10 of quartz sand away from the arcing zone and thus precludes the arc gap from being bridged by a fulgurite resulting from the fusion of quartz sand. Fig. 10 indicates the geometrical configuration of the two fulgurites F' resulting from blowing of such a fuse. The shape of the fulgurites F follows closely the configuration of the gaps formed between fuse link 6" and sleeve 12". In this embodiment of the invention transversal venting from the zone of neck 7" is entirely suppressed and venting is limited to axial or substantially axial flows of are products.

If sleeve 12" shown in Figs. 9 to 11 is substituted by a sleeve consisting of an inorganic material as, for instance, one of the inorganic materials used for making up are chutes in circuit breakers, the fuse has a strong tendency to fail, or restrike, due to the thermionic electron emission of such materials. Such an experiment shows conclusively the importance of applying synthetic resin binders in combination with silicious materials as, for instance, glass fibers for making up sleeve 12.

It will be understood that the plates 6 and 6' in the embodiments of the invention shown in Figs. 1 to 8 form flexible arc chutes capable of expanding and increasing the cross-sectional area of the venting passage for the products of arcing as the pressure due to arcing increases. The sleeve 12" in the structure of Figs. 9 to 11 forms a rigid arc chute, i. e. one which does not change its shape depending upon the pressure generated at the arcing zone.

The oscillograms shown in Figs. 12a and 12b evidence the effect of the presence of the plate structures 8 and 18' shown in Figs. 1 to 8, inclusive. Fig. 12a refers to a fuse of the same character as described above but without the plate structure 8 or 18. Such a fuse is disclosed, for instance, in United States Patent 2,781,434 to Kenneth W. Swain, Current-Limiting Fuses Comprising Fuse Links of Silver and Copper, Feb. 12, 1957, and reference may be had to that patent for a more detailed disclosure of such a fuse. The above patent refers to multimetallic fuse links, but substantially the same structure may be applied for fuses having links consisting of but one metal-i. e. silver or copperif the multimetallic feature is not desired. As shown in Fig. 12a a fuse having no arc-chute-forming plates or barriers fuses slightly earlier than a fuse having such plates or barriers. This is due to the heat absorbing capacity of the plate the fusing temperature slightly later thanin case of a similar fuse which has no arc chute structure on the-Surface of the link. The rate of current decay after fusion is considerably slower in the case of a fuse having no arc chute structure. This is due to the fact that a fuse lacking the particular are chute structure shown in Figs. 1 to 11, or an equivalent thereof, forms a fulgurite between the back-burning ends of the fuse link, whereas in the structures embodying this invention no such fulgurite is permitted to form, its place being taken by a gaseous arc gap. It will be apparent from a comparison of Figs. 12a and 12b that the arcing fi -dt of the fuses embodying this invention is much smaller than in the case of prior art fuses, such as those shown inthe above referred to Swain patent. This is very important for many applications as, for instance, the protection of germanium rectifiers and the protection of silicon rectifiers.

The provision of the link-destroying low-fusing-point metal overlay is highly desirable, yet its presence is optional rather than mandatory. If the overlay is present, the fuse may be caused to blow at temperatures below those at which the danger of thermal damage to the organic barrier or are chute structure 8, 18', 12" arises. The link-destroying low-fusing point metal overlay may, however, be omitted if the fuse is only applied jointly and in series with a circuit breaker including tripping means set to interrupt the circuit at currents less than the minimum current by which damage may be caused to the barrier or arc-chute structure 8, 18', 12".

Although this invention has been described in considerable detail, it is to be understood that such description is illustrative rather than limiting, as the invention may be variously embodied, and is to be interpreted as claimed.

I claim:

1. A small clearing ratio current-limiting fuse comprising a tubular casing of insulating material, a pair of conductive terminal elements closing the ends of said casing, a ribbon fuse link of a high conductivity low fusing energy metal conductively interconnecting said pair of terminal elements, said fuse link having at least one point of reduced cross-sectional area situated between the ends thereof, a pair of plates adapted to abstract large amounts of heat from the are formed at said point of reduced cross-sectional area, said pair of plates comprising silicious material in disperse form and an organic binder for said material, each of said pair of plates being arranged at a different side of in immediate heat-axchanging relation with said link and covering said point of reduced cross-sectional area thereof and defining paths for the escape of products of arcing from said point of reduced cross-sectional area substantially parallel to the plane defined by said link, a link-destroying low fusing point metal on said link adjacent said point of reduced cross-sectional area thereof, and a silicious pulverulent filler inside said casing substantially separated from said point of reduced cross-sectional area by said pair of plates.

2. A small clearing ratio current-limiting fuse comprising a tubular casing of insulating material, a pair of terminal elements closing the ends of said casing, a ribbon fuse link of a high conductivity low fusing energy metal conductively interconnecting said pair of terminal elements, said fuse link having at least one point of reduced cross-sectional area situated between the ends thereof to define a point of initial fusion and of arc initiation, a pair of are energy absorbing plates comprising a synthetic-resin-glass-cloth laminate physically engaging said point of reduced cross-sectional area to abstract are energy in the form of heat by virtue of the high latent heat of fusion of the glass cloth component of said laminate and a silicious pulverulent filler inside said casing substantially separated by said pair of plates from said point of reduced cross-sectional area of said link.

3. A small clearing ratio current-limiting fuse as specified in claim 2 wherein said point of reduced cross-sectional area of said link is provided witha link-destroying low fusing point metal overlay and wherein said pair of plates is made of a silicone-resin-glass-cloth laminate.

4. A small clearing ratio current-limiting fuse comprising a tubular casing of insulating material, a pair of conductive terminal elements closing the ends of said casing, a ribbon fuse link at least in part of silver conductively interconnecting said pair of terminal elements, said fuse link having at least one point of reduced crosssectional area situated between the ends thereof to define a point of initial fusion and of arc initiation, a linkdestroying low fusing point overlay on said link sub stantially at said point of reduced cross-sectional area thereof, a pair of relatively resilient plates of a syntheticresin-glass-cloth laminate arranged immediately adjacent said point of reduced cross-sectional area of said fuse link to abstract large amounts of heat from said point by virtue of the high latent heat of fusion inherent in the glass cloth component of said laminate, fastening means for said pair of plates projecting transversely across said pair of plates arranged relatively remote from the axially outer edges of said pair of plates to impart to said outer edges a relatively large freedom of resilient transverse motion, and a quartz sand filler inside said casing substantially spaced by said pair of plates from said point of reduced cross-sectional area of said link.

5. A small clearing ratio current-limiting fuse comprising a tubular casing of insulating material, a pair of conductive terminal elements closing the ends of said casing, a ribbon fuse link of a high conductivity low fusing energy metal conductively interconnecting said pair of terminal elements, said fuse link having at least one point of reduced cross-sectional area situated between the ends thereof to define a point of initial fusion and of arc initiation, a pair of relatively resilient insulating plates arranged on opposite sides of said fuse link and covering said point of reduced cross-sectional area thereof, fastening means integrating said fuse link and said pair of plates into a unitary structure, said fastening means being adapted to give a limited freedom of motion to the axially outer edges of said pair of plates to enable limited spreading of said edges in a direction transversely of said fuse link under the action of pressure generated at said point of reduced cross-sectional area of said fuse link, and a silicious pulverulent filler inside said casing substantially spaced by said pair of plates from said point of reduced cross-sectional area of said fuse link, said filler comprising portions disposed immediately adjacent said axially outer edges of said pair of plates.

6. A small clearing ratio current-limiting fuse comprising a tubular casing of insulating material, a pair of conductive terminal elements closing the ends of said casing, a ribbon fuse link of a high conductivity low fusing energy meta] conductively interconnecting said pair of terminal elements, said fuse link having at least one point of reduced cross-sectional area situated between the ends thereof to define a point of initial fusion and of arc initiation, an overlay of a link-destroying low fusing point metal on said point of reduced cross-sectional area of said fuse link, a pair of relatively resilient plates of a synthetic-resin-glass-cloth laminate arranged on opposite sides of said link and covering said point of reduced crosssectional area thereof, said pair of plates being adapted to form a pair of arc chutes establishing a pair of venting passages extending preponderantly in a direction longitudinally of said link to direct flows of products of arcing away from said point of reduced cross-sectional area of said link along said link preponderantly in a direction longitudinally thereof, and a silicious pulverulent filler inside said casing substantially spaced by said pair of plates from said point of reduced cross-sectional area of said link, said filler comprising portions disposed immediately adjacent the downstream ends of said pair of arc chutes in intimate contact with said link.

7. A small clearing ratio current-limiting fuse comprising a tubular casing of insulating material, a pair of conductive terminal elements closing the ends of said casing, a ribbon fuse link of a high conductivity low fusing energy metal conductively interconnecting said pair of elements, said fuse link having at least one point of reduced cross-sectional area situated between the ends thereof to define a point of initial fusion and of arc initiation, a link-destroying low fusing point overlay on said link substantially at said point of reduced cross-sectional area thereof, a pair of relatively flexible plates comprising an organic insulating material arranged on opposite sides of said link sufiiciently close to said point of reduced cross-sectional area to be directly subjected to the heat I of arcing upon fusion of said link, fastening means for securing said pair of plates to said link, said fastening means being situated relatively close to said point of reduced cross-sectional area to permit transverse bending of said pair of plates at the edge zones thereof under the action of pressure of products of arcing formed upon fusion of said link at said point of reduced cross-sectional area thereof, and a silicious pulverulent filler inside said casing substantially separated by said pair of plates from said point of reduced cross-sectional area of said link, said filler comprising portions disposed immediately adjacent said edge zones of said pair of plates.

8. A small clearing ratio current-limiting fuse comprising a tubular casing of insulating material, a pair of conductive terminal elements closing the ends of said casing, a ribbon fuse link of a high conductivity low fusing energy metal conductively interconnecting said pair of terminal elements, said fuse link having a point of reduced cross-sectional area sufficiently short in length and small in width to virtually form a point heat source, an 'overlay of a link-destroying metal having a relatively low fusing point on said link at said point of reduced crosssectional area thereof, a pair of relatively flexible plates of insulating material arranged on opposite sides of said link coextensive with said point of reduced cross-sectional area, fastening means projecting transversely across said pair of plates for securing said pair of plates to said link, said fastening means being arranged along a line extending transversely across said link and being relatively close to said point of reduced cross-sectional area to cause transverse bending of the edges of said pair of plates re mote from said line under the action of pressure of products of arcing formed in the region of said point of reduced cross-sectional area of said link, and a filler of quartz sand inside said casing substantially separated by said pair of plates from said point of reduced cross-sectional area of said link.

9. A small clearing ratio current-limiting fuse comprising a tubular casing of insulating material, a pair of conductive terminal elements closing the ends of said casing, a ribbon fuse link of a high conductivity low fusing energy metal conductively interconnecting said pair of terminal telements, said fuse link having a point of reduced cross-sectional area sufliciently short in length and small in width to virtually form a point heat source, a pair of relatively flexible discs comprising an organic insulating material arranged on opposite sides of said link with the center thereof substantially in registry with said point of reduced cross-sectional area, fastening means projecting transversely across said pair of discs for securing said pair of discs to said link, said fastening means being arranged in a straight line to minimize venting from the space between said pair of discs in the direction of said straight line, said pair of discs being free to separate at points remote from said straight line to form venting gaps therebetween substantially at an angle of degrees to said straight line, and a filler of quartz 9 10 sand inside said casing substantially separated by said 928,998 Murray July 27, 1909 pair of plates from said point of reduced cross-sectional 1,120,226 Murray Dec. 8, 1914 area of said link. 1,239,876 Burnham Sept. 11, 1917 10. A current-limiting fuse as specified in claim 9 2,326,031 Hodnette et a1. Aug. 3, 1943 wherein said point of reduced cross-sectional area of said 5 2,665,348 Kozacka Jan. 5, 1954 link is provided with an overlay of a link-destroying low 2,808,487 Jacobs Oct. 1, 1957 fusing point metal.

References Cited in the file of this patent UNITED STATES PATENTS 10 888,355 Schattner May 19, 1908 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,866,038 December 23, 1958 Frederick J, Kozaeka It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 22, for clearing" read eurrent limiting same line, for "current limiting" read m clearing column 4, line 16 for "silicone" reed silicioue column 6, line 49, for "heat-ax=-" read w heat fex o Signed and sealed this 2nd day of June 1959.

(SEAL) Attest:

KARL H AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,866,038 December 23, 1958 Frederick J Kozacka It is hereby certified that error appears in the -printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column l line 22', for "clearing" read m current same line, for "currentlimiting read clearing column 4, line 16,

for "silicous" read w silicious column 6, line 49, for "heat==ax=" read heat-=ex= a limiting Signed and sealed this. 2nd day of June 1959.

SEAL) Attest:

KARL Ho AXLINE.

Commissioner of Patents 

